The invention relates to digital multi-meter (DMMs) peripheral device cards.
FIG. 1 is a schematic diagram showing a conventional DMM. The DMM has a main body 2 which includes a rotary function selection switch 4, a negative input terminal 5, a positive input terminal 6 and a display screen 8. The function selection switch 4 is for switching the DMM on and selecting between different measurement functions, such as direct-current (DC) voltage, alternating-current (AC) voltage, resistance, DC current and AC current. The input terminals 4 allow the DMM to be connected via industry standard 4 mm banana connections to a pair of test probes 10. In operation the test probes are connected to a circuit or component under test. The display screen 8 is for displaying measurement information to a user, for example type of measurement being made and a numerical representation of the measurement value. In the example shown in FIG. 1, the display screen indicates that a DC voltage value of 0.3456 volts is present between the pair of test probes. The screen display for the DMM shown in FIG. 1 also includes a bar-graph type element for representing the measurement value as a percentage of a pre-determined range. This type of representation typically shows a measurement value with lower resolution than a numerical representation, but which can be updated faster.
FIG. 2 is a schematic diagram showing the internal circuitry of the conventional DMM shown in FIG. 1. The circuitry is powered by a battery of cells 14 which provide a voltage of Vbatt volts. The negative input terminal 5 is connected to the low voltage side of the battery of cells 14 and define a zero reference voltage V0. The high voltage side of the battery of cells is used to drive a reference voltage generating circuit 18 for generating positive and negative reference voltages V+ and V− respectively. The provision of both positive and negative reference voltages allows the DMM to measure positive and negative input voltages at the positive input terminal 6 with respect to the negative input terminal.
The position of the function selection switch 4 determines whether the positive input terminal is connected one of a DC voltage measuring circuit 20, a DC current measurement circuit 22 or a resistance measuring circuit 24. For simplicity, corresponding AC measuring circuitry is not shown in FIG. 2. The operation of each of the respective measuring circuits 22, 24, 26 is well known and not described here further. Each of the measurement circuits outputs an analogue voltage which is indicative of the measurement value of the measurement being made. The analogue voltage from the relevant measurement circuit is connected to an analogue-to-digital converter (ADC) 16, as determined by the position of the function selection switch 4. The ADC converts the analogue voltage to a digital signal, a representation of which can then be displayed on the display 8. Except for the input terminals, the internal circuitry of the DMM is fully insulated from the DMM housing by an insulation barrier 12. This is to prevent possibly hazardous internal voltages being accidentally applied to a user.
DMMs of the kind shown in FIGS. 1 and 2 have been around for several years with many commercial examples now available. Typically these come in a range of physical sizes ranging from comfortably pocket sized models to those which are better suited to workbench or desk use. A number of variations of the classic DMM are available, for example models with more or fewer measurement functions than shown in FIG. 1 or models with different measurement range and resolution. Some DMMs include the ability to store a number of measurements in an internal memory. The measurements can then be downloaded to a computer, normally via an RS232/RS422 or HPIB connection. These DMMs can typically take measurements at a few tens of samples per second. DMMs of this kind are useful and popular tools due to their portability and ease of use, however, their usefulness is hampered by the limited functional flexibility they provide, especially as regards how measurements can be viewed and stored.
With the rise in popularity of mobile computing, DMMs have become available in Personal Computer Memory Card International Association (PCMCIA) form-factor. These PCMCIA DMMs plug into industry standard PCMCIA slots in laptop computers. PCMCIA DMMs operate according to the same general principles as those described above for classic DMMs. However, PCMCIA DMMs have the advantage that they can use the display of the computer to present their readings in place of a dedicated screen display. This gives more freedom to a designer in how best to present measurement data to a user. However, this freedom comes at a cost in reduced portability over that of conventional DMMs. One example of a commercially available PCMCIA DMM card is the National Instruments NI PCMCIA-4050 PCMCIA card. This is a single channel DMM able to measure DC and AC voltages and resistance. It is able to measure DC and AC current with an appropriate external adapter. The card is a Type II format PCMCIA card and is 5 mm thick and measures approximately 85 mm×44 mm. The card has a hardwired flying-lead cable that terminates in two 4 mm banana sockets for connection to industry standard test leads.